The carotid arteries are the two large blood vessels on either side of your neck that supply oxygenated blood to your brain, eyes, and face. You can feel their pulse by pressing gently just below your jawline. Each one is roughly the diameter of a pencil in healthy adults, with the common carotid measuring about 6 to 6.5 mm across in men and slightly smaller in women.
Where the Carotid Arteries Are and How They Branch
You have one common carotid artery on each side of your neck. They travel upward alongside the windpipe until they reach roughly the level of the upper border of the thyroid cartilage (the bump often called the Adam’s apple). At that point, each common carotid widens slightly into a bulb called the carotid sinus, then splits into two branches: the internal carotid artery and the external carotid artery. This split is called the carotid bifurcation.
The internal carotid continues upward and enters the skull, where it delivers blood to the brain and eyes. The external carotid stays outside the skull and immediately begins branching off smaller arteries that feed the face, scalp, tongue, and throat. Its first branches supply the thyroid gland, the tongue, the face, the back of the head, and the throat lining.
What the Carotid Arteries Supply
The internal carotid artery is the more critical of the two branches. Once inside the skull, the two internal carotids join with the two vertebral arteries (which run up through the spine) to form a loop at the base of the brain called the circle of Willis. This loop acts as a distribution hub, ensuring blood reaches all major brain regions even if one vessel is partially blocked.
From there, the internal carotid gives rise to arteries that feed specific parts of the brain. The middle cerebral artery supplies the brain areas responsible for movement and sensation in the face and arms, as well as language centers that control speech production and comprehension. The anterior cerebral artery supplies regions that handle movement and sensation in the legs. A branch called the ophthalmic artery provides blood to the eye, the muscles that move the eye, the lacrimal (tear) gland, the upper nose, and parts of the forehead.
Because of this supply pattern, a blockage in one internal carotid can affect vision, speech, movement on one side of the body, or all three at once.
The Built-In Blood Pressure Sensor
The carotid sinus, that slight widening right at the bifurcation, does more than just funnel blood. It contains specialized pressure sensors called baroreceptors, stretch-sensitive nerve fibers embedded in the artery wall. These fibers constantly measure how much the vessel is being stretched by the force of blood flow, which is an indirect measurement of blood pressure.
When your blood pressure rises, the artery wall stretches more, and the baroreceptors fire off stronger signals up to the brainstem. In response, the brain dials down the sympathetic nervous system, causing blood vessels throughout your body to relax and your heart rate to slow. The net effect is a drop in blood pressure back toward normal. The reverse happens if pressure falls too low, such as during blood loss. The baroreceptors detect less stretch, and the brain ramps up heart rate, strengthens each heartbeat, and constricts blood vessels to push pressure back up. This reflex operates continuously and adjusts within seconds.
How Carotid Artery Disease Develops
The most common problem affecting the carotid arteries is atherosclerosis, a slow buildup of fatty plaque inside the artery wall. It begins when the inner lining of the artery gets damaged by factors like high blood pressure, smoking, or high cholesterol. Once the lining is disrupted, cholesterol particles (specifically LDL, the “bad” cholesterol) seep into the artery wall and become trapped. White blood cells follow, swallowing the cholesterol and swelling into what pathologists call foam cells.
Over time, smooth muscle cells from deeper in the artery wall migrate inward and multiply, adding bulk to the growing plaque. The foam cells and muscle cells together produce a fatty streak that pushes the artery lining inward. Eventually, a fibrous cap forms over the plaque, and the whole mass begins to narrow the channel blood flows through. This narrowing is called stenosis. The process can take decades before it causes any symptoms, which is why carotid artery disease often progresses silently.
Chronic inflammation keeps the cycle going. As the plaque grows, it encroaches further into the open channel of the artery, reducing blood flow to the brain. If the fibrous cap ruptures, a blood clot can form on the spot and either block the artery entirely or break loose and travel to the brain, causing a stroke.
Warning Signs of a Blocked Carotid
Carotid artery disease often produces no symptoms at all until the narrowing is severe or a piece of plaque breaks free. When warning signs do appear, they tend to be sudden and alarming.
One distinctive early warning is temporary vision loss in one eye, sometimes called amaurosis fugax. People describe it as a shade or curtain dropping down over their visual field. The episode is painless, typically lasts anywhere from a few seconds to about 30 minutes, and then resolves on its own. It happens because a tiny fragment of plaque or clot temporarily blocks blood flow through the ophthalmic artery to the retina.
Other warning signs include sudden weakness or numbness on one side of the body, difficulty speaking or understanding speech, and dizziness. These symptoms overlap with a transient ischemic attack (often called a mini-stroke), which occurs when blood flow to part of the brain is briefly interrupted. A TIA resolves within minutes to hours, but it signals that a full stroke could follow.
How Carotid Narrowing Is Measured
The standard first-line test is a carotid ultrasound, a painless scan that uses sound waves to image the artery and measure how fast blood is flowing through it. Narrower arteries force blood to accelerate, so the speed of flow indicates the degree of blockage. In a healthy carotid, peak blood flow velocity stays below 125 cm per second. When narrowing reaches 50 to 69%, flow speeds rise to between 125 and 230 cm/sec. At 70% narrowing or more, speeds exceed 230 cm/sec. A completely blocked artery shows no detectable flow at all.
These velocity measurements, combined with the visual image of the plaque itself, give doctors a reliable picture of how severe the disease is without any needles or dye injections.
Treatment Thresholds
Not every narrowed carotid artery needs surgery. The decision depends on how much the artery is blocked and whether the person has experienced symptoms like a TIA or stroke.
For people who have had symptoms, surgical guidelines recommend a procedure called carotid endarterectomy (where a surgeon opens the artery and removes the plaque) when narrowing exceeds 50%, ideally within two weeks of the last episode. The urgency reflects the high short-term risk of a full stroke after a TIA. For people without symptoms, the threshold is higher: surgery is typically recommended only when narrowing reaches 70 to 99%, and mainly in men under 75, provided the surgical team’s complication rate is very low.
An alternative procedure, carotid stenting (where a mesh tube is threaded into the artery to hold it open), is generally reserved for people who are poor candidates for open surgery, or in specialized centers with documented low complication rates.
For narrowing that doesn’t meet these thresholds, treatment focuses on controlling risk factors: managing blood pressure and cholesterol, quitting smoking, and taking medications that reduce the chance of clot formation. These measures can slow plaque growth and, in some cases, stabilize plaque so it’s less likely to rupture.